Conventionally, a charging device such as a turbocharger is applied to an internal combustion engine for enhancing efficiency of intake air, thereby increasing engine power. According to JP-A-7-332097, a charging control apparatus is provided to an engine for controlling a charging device. A bypass passage is provided to bypass an exhaust turbine provided to an exhaust pipe. A waste gate valve is provided to the bypass passage for controlling an amount of exhaust gas flowing into an exhaust turbine, thereby controlling charging pressure.
In general, a waste gate valve is controlled using a positive pressure actuator. A structure of the waste gate valve is described in reference to FIG. 10. A turbocharger 80 includes an intake compressor 81 and an exhaust turbine 82. The intake compressor 81 is provided to an intake pipe 85. The exhaust turbine 82 is provided to an exhaust pipe 86. The exhaust pipe 86 connects with a bypass passage 87 that bypasses the exhaust turbine 82. A waste gate valve (WGV) 88 is provided to the bypass passage 87.
The WGV 88 connects with an actuator 90 that has a pressure chamber 92 partitioned with a diaphragm 91. The WGV 88 is operated in accordance with pressure in the pressure chamber 92. Intake pressure (charging pressure) downstream of the intake compressor 81 is applied to the pressure chamber 92 through a pipe 93. A pressure control valve (vacuum switching valve) 94 is operated using a microprocessor or the like, so that pressure applied to the pressure chamber 92 is controlled.
When the pressure control valve 94 is closed, charging pressure is directly applied to the pressure chamber 92, so that the WGV 88 is operated in accordance with the charging pressure. Specifically, as charging pressure becomes high, pressure in the pressure chamber 92 becomes high. In this condition, the WGV 88 is operated on the opening side thereof, so that motivity of the exhaust turbine 82 decreases. Thus, motivity of the intake compressor 81 also decreases, so that charging pressure decreases. By contrast, as the pressure control valve 94 opens, pressure applied to the pressure chamber 92 decreases. Therefore, pressure in the pressure chamber 92 does not increase even when charging pressure becomes high, so that the WGV is maintained in a close position. Thus, the motivity of the turbine is maintained even when charging pressure increases. As a result, charging pressure is maintained, or is increased.
However, in this conventional structure, in general, when charging pressure increases, the WGV 88 is opened, so that charging pressure is restricted from excessively increasing. Specifically, the WGV 88 is maintained in the close position in a normal condition, and the WGV 88 slightly opens in one of a high load condition and a high rotation speed range. In this condition, the position of the WGV 88 is restricted in dependence upon the charging pressure. Accordingly, fuel efficiency cannot be sufficiently enhanced in a supercharging operation using a turbocharger.